This invention is related to video disc systems, and is particularly concerned with improving the performance and reliability of a CED (capacitance electronic disc) system.
In the CED system, the video and audio information is embodied in a spirally grooved disc. Playback is by a pick-up stylus similar to the stylus of an audio record player; however, the information density is much greater in the CED as there are forty grooves in the video disc system for every one in an audio system. The video information on the capacitance electronic disc is in the form of successive dimensionally varying information track elements in the bottom of each groove. These elements induce a varying capacitance between a metallized electrode on the pickup stylus tip, which serves as one plate of a capacitor, and the disc, which serves as the other plate. The signal on the disc is in the form of frequency modulated video and audio carriers. The elements very minutely; e.g., in the order of Angstrom units, specifically, about 800 .ANG. peak-to-peak. Hence the variance in capacitance provided is also very small, with a capacitance change of about 10.sup.-4 picofarad.
The signal induced by this very small capacitance is coupled to a 910 MHz tuned line which is driven by a 915 MHz oscillator, resulting in amplitude modulation of the 915 MHz signal. An amplitude detector circuit provides an FM carrier output signal of five to ten millivolts peak to peak. The signal spectrum of the video disc comprises an audio carrier at 716 kHz and a video FM carrier with side bands from 2 to 9.3 MHz.
The CED system also includes means to detect and correct defects in the signal. When the defect detector is activated, a corresponding portion of the previous horizontal line of information is inserted into the area of the defect. By this process, momentary defects in the video carrier are not displayed; instead, the corresponding part of the previous horizontal line is displayed. As many as three horizontal lines can be inserted by the defect corrector before there is any noticeable degradation in the television picture.
The CED system is immune to normal sources of interference such as the local oscillator signals of nearby radio and television sets, citizen's band and general communication signals, and RFI from nearby appliances. However, the very small signal output, coupled with the fact that the tuned line operates at a frequency of 915 MHz (within the "garbage" band), make this CED system vulnerable to an unusual source of radio frequency interference. This interference originates in a radar set in general use. The radar set emits high power pulses at the rate of several hundred per second in the frequency band of interest, at a scan rate of six or twelve revolutions per minute. The pulses are detected by the CED system and appear as superimposed on the envelope of the 5 MHz signal which carries the video information. The pulses are clearly visible in the television picture.
High-density capacitive information records and playback apparatus therefore is disclosed in U.S. Pat. No. 3,783,196 to Stanley.
A circuit is known which responds to radio-frequency interference of the type originating from the aforedescribed radar set. This relatively complex circuit utilizes an integrated circuit synchronous detector for detecting short duration noise pulses on the envelope of a video FM carrier. Upon receipt of such pulses, a transistor switch closes to activate a substitution system which blanks that portion of the raster line that would otherwise display the noise pulse, and interjects the corresponding part of the previous line. The circuit also includes means for suppressing sync pulses that appear on the envelope of the video FM carrier. The relative complexity of the circuit, and the large number of components render it undesirably slow in response time, and relatively expensive to manufacture.